Star = Name of the star around which the object orbits (it can be any of the star's names, ex : Alpheratz, Sirrah, ALF And, 21 And, DEL Peg, HD 358, HIP 677).

To make an object orbit a planet, you only need to add /Planet after the name of your star (for example : Sol/Earth). To make it orbit a moon you will have to add /Moon after Star/Earth (for example : Sol/Jupiter/Io) and so on.

2) The 5 classes are : planet, moon, comet, asteroid, and spacecraft

They differ in the displayed color of the name, the possibility of being able to see the orbits, like in the case of a planet or a moon. If you want to make a space ship and be able to see it’s orbit without selecting it you may change it’s class to a moon, for example.
The class is not a required field, it is possible that Celestia may retrieve it for himself by looking at the radius of the object.

3) With this line set to true, the object is considered as a light source whose colour depends of the original colours of the differents parts of the objects.

Ex :

Hubble without Emissive	and with...

4) Generally one does not use the functions Textures and Mesh together because the models are already textured, the Texture function is thus useful only for the planets and moons, spherical by default.

The extensions possible for the textures files are : jpg, bmp, tga, avi, dds, png.

All the files must come in a format in which the size of the image is multiple of 2, example: 2048x1024, 8192x4096, etc, or else Celestia will be unable to read the files and you shall obtain a beautiful white, blue or purple planet !

By default Celestia will access the textures files that are located on the textures/medres directory and then in the lowres and hires directory if you press ‘r’ or ‘shift+r’.
If you want to use textures of different qualities on Celestia and you have one in a resolution of 4096x2048 for example, place that file on the hires directory and then reduce the image resolution, with an editor of your choice, to obtain an image in 2048x1024 which you’ll place in the medres directory and one in 1024x512 in the lowres directory. All the files must have the exact same name.

5) This line may be used with any kind of class although it is not meant for the planets and the moons that possess a spherical geometry (an exception are objects like the two small satellites Phobos and Deimos, for example).

The possible extensions are 3ds and cms. I do not know much about the cms files that appear to only be used for the models of the asteroids but I wont say the same about the 3ds files. This format is used by applications like 3D Studio Max, you have to have a copy of this kind of software in order to be able to edit or create these files and then export the model onto the 3ds format. If you want to texture your models you must place your textures (always in a format multiple of 2: 64x128, for example) on the medres directory and then apply them to your model. I advise you to, when using 3dsmax, not to use the multiple surfaces textures that are not supported upon exporting the model to 3ds, and use the UVW Map modifiers to correctly map the textures or else you’ll end up with very lousy models once you see them in Celestia (believe me!)

6) NightTexture like the name says is used as the night textures of the planets. These textures appear progressively as the surface stops being illuminated by the sun.

Earth without NightTexture	and with ....

7) The BumpMap function is used to simulate the planet’ s relief from black and white images representing the altitude of different points of the globe. These file are limited to a size of 2048x1024 and wont work with textures of the dds format. It seems that this function only works with Nvidia-type video cards (like GeForce).

Mercure without BumpMap	and with....

8) If the restrictions above are respected, the function BumpHeight allows then the modification of the altitude of the planet’s relief. This value is approximately equal to 2 by default.

9) This corresponds to the color of planet when you see it by far, and closer when no texture are assigned to it. This color is coded according to the system [ Red Green Blue ] with a value ranging between 0 and 1 for each one of these components, for example for an entirely red planet these values will be : [ 1 0 0 ] and for the Earth : [ 0.85 0.85 1.0 ].

10) SpecularColor corresponds to the color of the reflections of a star on the surface of a
planet, it is coded in the same manner as the Color function. Moreover these reflections are conditioned by the presence of an alpha layer in the texture of the surface of planet, which then makes it possible to obtain reflections much more realistic which are limited to the surface of the oceans and the seas.

SpecularColor [0.05 0.5 0.55]	SpecularColor [1.0 0.0 0.0]

11) SpecularPower makes it possible to manage the importance of these reflections, approximately it intervenes on the size of the luminous spot that it sees on the surface, with 0 the spot occupies all surface then when it's 100 it occupies a place much more restricted. On the Earth the value is fixed at 25.

12) HazeColor must be an option to manage a kind of fog on the surface of planet in order to recreate the effects of the atmosphere but it seems that it only works with a Geforce3 or other more powerful cards thus I’m sorry, it will be necessary to wait a little before knowing what it does ;)

13) Same thing as above.

14) Radius of the object, in kilometers. For the 3ds files, the models are all resized to fill in a cube with a side unit then their size is determined by the value of the "radius".

15) Oblateness corresponds to the "flatness" of the planet, it is equal to 1-(radius at the pole/radius at the equator). Thus for a value of 0 you have a spherical planet and for a value of 1 you obtain a disc! (for the Earth, Oblateness=0.003). Attention however because the atmosphere is not modified and thus does not follow the new geometry of the planet.

Oblateness=1	Oblateness=0.66	Oblateness=0.33

17) Thickness in kilometers of the atmosphere, this one is visible at low altitude on the
periphery of planets in the form of a colored halation.

18) and 19) The parameters Lower and Upper corresponds respectively to the colors of the low and high part of the atmosphere and thus makes it possible to create a gradient from the surface to the space.

20) Sky corresponds to the colour of the sky as you would see it if you were situated somewhere below the altitude of the atmosphere given in "Height".

Example of how to use these elements :

Height 100
Lower [0.0 0.0 1.0] # Bleu
Upper [0.0 1.0 0.0] # Vert
Sky [1.0 0.0 0.0] # Rouge

6000 km	2000 km	500 km	100 km	80 km	50 km	20 km

21) Altitude in kilometers of the clouds.

22) Speed, measured in kilometers per hour, of the clouds.

23) Texture of the clouds, these textures must have an alpha layer making it possible to see surface by transparency or they mask this texture as on Venus where the clouds are so dense that they hide the surface of planet. It is thus necessary to avoid the files JPEG which do not manage the alpha layers but to rather use png or dds files. Apparently it would seem that textures of clouds can't exceed the resolution of 2048x1024, when using an 8k cloud texture I realized it wasn't showing more details than if it was a 2k one.

27) Period and SemiMajorAxis are the only absolutely compulsory orbital elements, if not
Celestia cannot create the new object.

For planets the period is counted in terrestrial years (1.00 for the Earth, 0.6152 for Venus and 248.54 for Pluto) whereas for the satellites it is counted in terrestrial days (27.32 for the Moon).

28) SemiMajorAxis corresponds to the half large axis of the orbit of the object, this value is connected to the period by the formula of Kepler : (T²)/(4p²)=(a^3)/(G*M)

with T Period of the object

a Semi major axis of its orbit (in meters)

G Constant of gravitation= 6.67*10^-11

M Mass of the star around whose the object is in rotation (in kg)

For the planets the semi major axis is counted in astronomical units (UA = 150 million km) whereas for the satellites it's in km.

### The following values correspond to the orbital elements of the object, they are necessary only if you plan to give a precise orbit to your object or if not you can leave it blank, Celestia will give him an orbit by default.

If you want nevertheless to fill these fields, two choices are given then: either you have the real data of your object and it is then enough to rewrite them or make them up, but then a small explanation is imposed (as I do not claim to be an expert on the matter I have recopied these explanations and recovered a diagram). ###

29) The eccentricity is a value ranging between 0 and 1 making it possible to define the elliptic form of an orbit, with 0 the orbit is not deformed it is thus a circle and this orbit is
increasingly elliptic when it approaches 1.

30) The inclination measured in degrees corresponds to the orientation of the plane of the orbit compared to the plane of the terrestrial equator.

31) The right ascension of the ascending node (in degrees) which determines the
orientation of the axis of the nodes compared to a direction of reference (vernal point).
In practice, one often positions the plane of the orbit starting from the longitude of the
ascending node on a given date (the ascending node is the point of intersection of the orbit with the plane of the equator when the satellite goes up in the southern hemisphere towards northern hemisphere).

32) The argument of the perigee (in degrees) gives the position of the axis of the ellipse compared to the equatorial plane. It moves the angle, in the plane of the orbit, between the line of the node (which belongs to the plane of the equator) and the large axis of the ellipse.

33) The average anomaly corresponds to the position of the satellite in its orbit around the Earth compared to the axis of the perigee.

34) Epoch of the orbital elements. It is counted in days and decimal fractions of days, knowing that to the epoch by default, January 1, 2000 at noon, corresponds the value 2451545.

And if this diagram is not enough here is the url of a site containing all the
essential data on these orbital elements: http://spaceflight.nasa.gov/realdata/elements/

36) Period of rotation of the object around itself in hours.

37) Obliquity corresponds to the slope of the axis of rotation of the object.

38) Longitude of the axis of rotation projected on the orbital level.

"Normal" 3D base	After exporting in Celestia	Obliquity and LongOfRotationAxis

39) Rotation of the object at the time given by the epoch (1st Jan 2000 by default).

40) PrecessionRate corresponds to the rate of precession of the axis of rotation in rad/days.

### If, as me, you haven't really understood the utility of these last functions you can
also thank Matt McIrvin from the Celestia forum
(http://www.shatters.net/forum/viewtopic.php?t=427) for finding the tip allowing me to have a satellite always pointing in the same direction on the surface of the Earth (like the geostationary satellites). You only have to copy the value of Inclination for Obliquity and that of AscendingNode for LongOfRotationAxis and your satellite points then always in the same direction compared to the Earth, then you only have to play with the value of RotationOffset to have it pointing in the right direction ! ###

41)The albedo determines the fraction of luminous power reflected by an enlightened object. If this object is dark it will reflect less light and its albedo is weak, close to 0. On the contrary, a clear object will reflect more and thus have a greater albedo. In Celestia this value influences especially the visibility of the objects seen by far, as a point. For example if you want your satellite to be visible farter than 10 km you just have to increase its albedo.

43) et 44) Inner and Outer radius of the rings.

45) Textures of the rings : it corresponds to the visible and "invisible" area of the rings, like for the clouds. It appears as a section of the disc (it can have a size of 512x2 for example) and is thenapplied by rotation to the whole rings.

Examples of rings :

(squares are transparent areas)

46) Rings color, in [R G B] system. In the example above, it corresponds to the Saturn's rings color : [ 1.0 0.88 0.82 ]